1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be installed in an exhaust system of an internal combustion engine for air-fuel ratio control, and more particularly to an improved structure of a mechanical seal which keeps a reference gas chamber and a gas chamber airtight in a gas sensor.
2. Background Art
Gas sensors are known which are fabricated by inserting a sensor element into an insulation porcelain, mounting the insulation porcelain in a housing, installing a gas cover and an air cover on a front end and a base end of the housing, respectively, and sealing the gap between the insulation porcelain and the housing hermetically. This seal defines a measured gas chamber and an air chamber within the gas sensor.
The sensor element has a measuring electrode exposed to a gas to be measured and a reference electrode exposed to a reference gas or air and provides a signal in the form of an ion current flowing through the measuring and reference electrodes or a potential difference between the measuring and reference electrodes to determine the concentration of the gas. The leakage of the gas from the measured gas chamber to the air chamber will, thus, result in a decrease in accuracy of measuring the concentration of the gas. In order to avoid this problem, typical gas sensors pack powder material such as talc in the gap between the insulation porcelain and the housing to separate the measured gas chamber and the air chamber hermetically.
The use of powder material such as talc, however, results in an economical disadvantage that the pressure required to pack the powder material and the amount of powder material must be controlled finely and precisely.
In order to alleviate such a drawback, bulk material-made packing is proposed as a sealing member. For example, U.S. Pat. No. 5,795,454 teaches a ceramic ring baked at lower temperature for sealing a gap between a sensor element and a housing to define a measured gas chamber and a reference gas chamber hermetically. The ceramic ring, however, usually remains having a certain degree of porosity even after the ceramic ring is installed under high pressure, which may result in lack of airtightness between the sensor element and the housing.
U.S. Pat. No. 5,795,454 also discloses use of a lower porosity metallic ring together with the ceramic ring for increasing the degree of the airtightness, however, it will result in increases in fabrication process and manufacturing cost. Moreover, the metallic ring may corrode early depending upon the type of a gas to be measured, which leads to a decrease in degree of the airtightness between the measured gas chamber and the reference gas chamber.
U.S. Pat. No. 5,795,454 further discloses a metal seal coated with nickel or copper, however, it requires plating or cladding the whole surface of the metal seal, thus resulting in an increase in manufacturing cost.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an improved structure of a gas sensor which provides a mechanical seal required to keep a reference gas chamber and a gas chamber in the gas sensor airtight highly.
According to the first aspect of the invention, there is provided a gas sensor which features a mechanical seal and which may be installed in an exhaust system of an internal combustion engine for air-fuel ratio control. The gas sensor comprises: (a) a hollow housing having a sealing shoulder formed on an inner wall thereof; (b) a sensor element having a length which includes a first and a second portion; (c) an insulating member disposed within the housing, retaining the sensor element therein hermetically, the insulating member having a sealing surface formed thereon; and (d) a metal sealing member interposed between the sealing shoulder of the housing and the sealing surface of the insulating member to hermetically define a first chamber in which the first portion of the sensor element is disposed and a second chamber in which the second portion of the sensor element is disposed. The metal sealing member is made of at least one of a nickel, a nickel alloy, a titanium, and a stainless steel.
In the preferred mode of the invention, the first chamber leads to the atmosphere. The second chamber is so constructed as to admit a gas to be measured thereinto.
An air cover is installed on an end of the housing to define the first chamber therein.
The metal sealing member has a Vickers hardness of 200 or less and a thickness of 0.1 mm or more.
The metal sealing member may be made up of a first layer and a second layer. The first layer is in contact with the sealing shoulder of the housing. The second layer is in contact with the sealing surface of the insulating member and has a hardness lower than that of the first layer. The second layer has a Vickers hardness of 200 or less.
The sealing surface of the insulating member is tapered. The sealing shoulder of the housing is so oriented as to adhere to the sealing surface of the insulating member through the metal sealing member.
The insulating member is made of an alumina ceramic having an alumina purity of 90% or more.
The sealing surface of the insulating member has a ten-point average roughness of 10 xcexcm or less.
According to the second aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow housing having a sealing shoulder formed on an inner wall thereof; (b) a sensor element having a length which includes a first and a second portion; (c) an insulating member disposed within the housing, retaining the sensor element therein hermetically, the insulating member having a sealing surface formed thereon; and (d) a metal sealing member interposed between the sealing shoulder of the housing and the sealing surface of the insulating member to hermetically define a first chamber in which the first portion of the sensor element is disposed and a second chamber in which the second portion of the sensor element is disposed. The metal sealing member has a Vickers hardness of 200 or less.
According to the third aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow housing having a sealing shoulder formed on an inner wall thereof; (b) a sensor element having a length which includes a first and a second portion; (c) an insulating member disposed within the housing, retaining the sensor element therein hermetically, the insulating member having formed thereon a sealing surface having a ten-point average roughness of 10 xcexcm or less; and (d) a metal sealing member interposed between the sealing shoulder of the housing and the sealing surface of the insulating member to hermetically define a first chamber in which the first portion of the sensor element is disposed and a second chamber in which the second portion of the sensor element is disposed.
In the preferred mode of the invention, the sealing surface of the insulating member is polished.
The sealing surface of the insulating member may alternatively be plated.
The insulating member is made of an alumina ceramic having an alumina purity of 90% or more.
According to the fourth aspect of the invention, there is provided a gas sensor which comprises: (a) a hollow housing having a sealing shoulder formed on an inner wall thereof; (b) a sensor element having a length which includes a first and a second portion; (c) an insulating member disposed within the housing, retaining the sensor element therein hermetically, the insulating member having a sealing surface formed thereon, the insulating member being made of an alumina ceramic having an alumina purity of 90% or more; and (d) a metal sealing member interposed between the sealing shoulder of the housing and the sealing surface of the insulating member to hermetically define a first chamber in which the first portion of the sensor element is disposed and a second chamber in which the second portion of the sensor element is disposed.